1. Field of the Invention
The present invention relates to an imaging lens, and more particularly to an imaging lens which is suitable for installation into an image input device of a portable telephone or personal computer, a digital camera, a CCD (charge-coupled device) camera used for monitoring purposes, a surveying device, or similar which uses a CCD or CMOS (complementary metal-oxide semiconductor) as an imaging device.
2. Description of Related Art
In such an imaging lens, the optical length, which is defined as the distance from the entrance surface on the object side of the imaging lens to the imaging surface (the image-forming surface of a CCD or the like), must be short. In other words, during design of the lens, a method of reducing the ratio of the optical length to the combined focal length of the imaging lens is required. An imaging lens having a short optical length and a small optical length to focal length ratio will occasionally be referred to as a compact lens below.
Taking a portable telephone as an example, the optical length must at least be shorter than the thickness of the portable telephone body. Meanwhile, the back focus, which is defined as the distance from the exit surface on the image side of the imaging lens to the imaging surface, is preferably as long as possible. In other words, during design of the lens, a method of increasing the ratio of the back focus to the focal length as much as possible is required. This is due to the need to insert components such as a filter or cover glass between the imaging lens and the imaging surface.
As well as the points described above, there is also demand for imaging lenses in which various types of aberration have been corrected to such a small degree that image warping is not visually recognizable and that is required by the integration density of imaging elements (also known as “pixels”). In other words, favorable aberration correction is required, and images in which such aberration correction has been performed favorably will occasionally be referred to as “favorable images” below.
As will be described below, imaging lenses with a three-layer structure which are suitable for use in imaging devices such as portable computers, video telephones, or similar using a solid-state imaging device such as a CCD or CMOS have been disclosed. These lenses all secure a wide viewing angle, and are compact and lightweight.
Of these lenses, an imaging lens capable of obtaining images with favorably corrected aberration while securing a wide viewing angle has been disclosed as a first three-layer lens (see, for example, Japanese Unexamined Patent Application Publication 2001-075006).
However, the refractive power of these three lenses, which are constituted by first, second, and third lenses arrayed in succession from the object side, is positive in the first lens, negative in the second lens, and positive in the third lens, and hence the distance (optical length) from the surface of the first lens on the object side to the imaging surface is too long. Further, a diaphragm is disposed on the object-side surface of the first lens, and hence the effective diameter of the third lens cannot be reduced. As a result, a compact lens cannot be produced.
Imaging lenses in which aberration is favorably corrected and a short focus is realized while securing a wide viewing angle have been respectively disclosed as second through fourth three-layer lenses (see, for example, Japanese Unexamined Patent Application Publication 2003-149548, Japanese Unexamined Patent Application Publication 2002-221659, and Japanese Unexamined Patent Application Publication 2002-244030).
However, similarly to the imaging lens described above, the refractive power of the three lenses of these imaging lenses, constituted by first, second, and third lenses arranged in succession from the object side, is positive in the first lens, negative in the second lens, and positive in the third lens. Hence, although these imaging lenses are set with a short combined imaging lens focal length, the back focus is long, and thus the optical length is too long. In addition, these lenses use glass materials, and are therefore expensive.
An imaging lens which uses aspheric lenses and is reduced in size by appropriately setting power distribution and surface shape has been disclosed as a fifth three-layer lens (see, for example, Japanese Unexamined Patent Application Publication 2003-149545).
However, the refractive power of the three lenses of this imaging lens, constituted by first, second, and third lenses arranged in succession from the object side, is negative in the first lens, positive in the second lens, and negative in the third lens. As a result, the imaging lens has a long optical length. In addition, the lenses use glass materials, and are therefore expensive.
A lens in which a pair of meniscus lenses whose concave surfaces face each other are constituted by plastic lenses each having at least one aspheric surface, and in which the entire lens system has a three-layer structure, has been disclosed as a sixth three-layer lens (see, for example, Japanese Unexamined patent Application Publication H10-301022). This lens achieves compactness and low cost, and is capable of suppressing focus movement due to temperature change with ease.
However, the refractive power of the three lenses in this imaging lens, which are arranged as first, second, and third lenses in succession from the object side, is weak in the first lens, weak in the second lens, and positive in the third lens, and hence the refractive power of the first lens and second lens cannot be fully compensated for by the third lens alone. As a result, the back focus lengthens, causing an increase in the optical length. Furthermore, the third lens uses a glass material, and hence cost reduction is incomplete.
A low-cost lens system with a short optical length which has a telephoto-type lens constitution in which the entire lens system is divided into a front group and a rear group, the front group having a positive refractive power and the rear group having a negative refractive power, has been disclosed as a seventh three-layer lens (see, for example, Japanese Unexamined Patent Application Publication H10-301021).
However, the refractive power of the three lenses in this lens system, which are arranged as first, second, and third lenses in succession from the object side, is negative in the first lens, positive in the second lens, and negative in the third lens, and the distance between the second lens and third lens is wide. As a result, the optical length is long, and the aperture of the third lens widens. This is unsuitable for installation in image input devices of portable telephones or personal computers, digital cameras, CCD cameras used for monitoring purposes, surveying devices, and so on.
An imaging lens comprising, in succession from the object side, two positive lenses, and a negative lens whose concave surface faces the image side, both surfaces of which are aspheric and the negative power of which gradually weakens from the center of the lens toward the periphery so as to have a positive power on the periphery, has been disclosed as an eighth three-layer lens (see, for example, Japanese Unexamined Patent Application Publication 2003-322792).
In this lens system, however, the lens corresponding to a third lens L3 gradually weakens in negative power from the center of the lens toward the periphery, and the position where the negative power turns into positive power exists within a range of between 0.7 times and 1.0 times the effective diameter of the lens from the center of the lens. In the lens disclosed in the embodiments of the invention, the positions where the negative power turns into positive power are set respectively at 0.96 and 0.97 times the effective diameter of the lens from the center of the lens, i.e. substantially at the periphery of the lens.
By setting the position where negative power turns into positive power at the peripheral portion of the lens, light entering the vicinity of the intersecting point between the optical axis of the lens and the imaging surface and the periphery of the lens has an almost right-angled angle of incidence onto the imaging device, whereas in an intermediate position between the intersecting point of the optical axis of the lens and the imaging surface and the periphery of the lens, the angle of incidence onto the imaging device deviates greatly from a right angle. Since the angle of incidence of the light entering an intermediate position from the peripheral portion of the lens, which forms an important part of an image, deviates greatly from a right angle, the light enters the imaging device in a diagonal direction to the imaging device, thereby increasing the amount of reflection on the entrance surface such that the light reaching a photoelectric conversion surface of the imaging device is low in energy. As a result, this part of the image becomes dark.
It is therefore an object of the present invention to provide an imaging lens which is suitable for installation in a camera using a CCD or CMOS as an imaging device, which has a short optical length (a small optical length to focal length ratio), a back focus which is as long as possible (a back focus to focal length ratio which is as large as possible), and which is thus capable of obtaining favorable images.
A further object of the present invention is to provide an imaging lens in which all of the (three) lenses constituting the imaging lens of the invention are made of plastic materials to thereby reduce cost and weight. Here, “plastic materials” refers to high polymeric substances which are transparent to visible light, and may be molded by being subjected to plastic deformation through application of heat, pressure, or both, and thereby formed into lenses.